In motor vehicles, to measure the degree of wetting (i.e., moisture) present in the wiper field of windshields or rear windows, a rain sensor is installed on the inner side of the window. If the wiping system is switched on in the automatic operating mode, and the window is wetted with moisture at the location where the sensor is installed, a wiping operation is initiated.
The mode of operation of the sensor is based on an optical principle. In particular, a light beam of known intensity emitted by a transmitter is introduced into the pane at a certain location and is directed through a defined length of the pane by means of repeated total reflection within the pane. At a second selected location, the light beam is directed by means of optical coupling out to a receiver and to a measuring system where the residual light intensity is measured and analyzed.
When the surface of the pane is wetted with drops of water, part of the light beam is no longer completely reflected, but rather emerges from the pane. The resultant loss of intensity of the residual light is a measure of the pane surface wetting. Below a preselected threshold value, a means for cleaning the window pane is then automatically activated.
Rain sensors of this type are often installed in motor vehicles at the level of the rearview mirror to ensure that the driver's vision is not impaired by the sensor housing. In some motor vehicles, however, a light filter, e.g., a green or gray wedge tinting the upper edge of the pane, is situated at this location and is integrated in the windshield. When the wave length of the transmitter beam being used is within a range that is essentially attenuated by the light filter, it is not possible to position the optical rain sensor in the light filter, since the repeated total reflection in the pane causes the light beam to pass through the light filter several times, attenuating it to an unacceptable degree.